For large dish antenna, such as those used for radio astronomy and satellite communication, high precision is required in order to pick up weak signals and to separate desired signals from noise In order to obtain such precision, it is necessary that the antenna in general, and the antenna dish in particular, be carefully tested before being initially installed, and periodic testing may be required thereafter. Such testing has generally been performed on a test range. However, the Rayleigh distance required for such range is given by the equation R.dbd.2.sup.D.sup.2 /.lambda., where .lambda. is the wave length of the test signals and D is the diameter of the antenna under test. Thus, for large antenna, with diameters in the range of 25ft or more, the Rayleigh distance becomes prohibitively large for available test ranges. At these distances, echoes from ground reflections and the like as well as other noise are also problems.
To overcome these problems, attempts have been made to test antenna by receiving transmissions from satellites or from stars. However, to fully map the characteristics of an antenna dish may take days, making it difficult to perform such measurements with stars or satellites. Further, when operating in open space, wide variations in environmental conditions such as temperature, humidity, wind, clouds, and the like make it difficult to correlate readings taken over an extended period of time and to obtain high precision test results from such data.
To overcome the problems indicated above, a technique has been developed to take near-field measurements in a controlled environment, and to then mathematically transform the near-field measurements to obtain far-field values. Utilizing near-field measurements is advantageous since it permits measurements to be taken in an enclosed chamber where environmental conditions may be carefully controlled. For example, temperature can be controlled within .+-.5.degree., humidity can be controlled within similar limits, wind can be eliminated and stray electric and magnetic fields can be shielded. In addition, the chamber can be covered with RF signal absorbing material to substantially reduce noise resulting from RF reflections. The reflectivity of the chamber is typically about -45 dB, and this number can be improved where required. Near-field measurements also permit the use of a transmitting source which may be selected to transmit at the optimum frequency for the antenna under test and permit tests to be conducted at times desired by the user and over-extended periods of time. This permits far more accurate measurements then can be obtained with existing far-field measuring systems.
However, in order to fully map an antenna dish, three types of movement are required between the probe and the antenna. The first of these movements is movement in the azimuthal or phi(.phi.) axis or direction. The second is movement in elevation or theta(.theta.) axis or direction. The third axes is the polarity or chi(.chi.) axis. Taking measurements in all three of these directions requires certain movements of the antenna and probe. A number of apparatus exist or have been proposed to achieve the desired movements, most of which involve moving the probe in at least one direction by use of some type of robotic arm. While this is feasible for measurements of relatively small antenna, for a large antenna the extent of such arms become too long for practical use. Particularly with long arms, it is also difficult to obtain the precise positioning required in order to accurately map the antenna dish.
Further, particularly for large antenna, the alignment of the panels forming the antenna dish change slightly as a result of gravity as the antenna is moved to different elevations. Typically, the panel alignment is prebiased for a particular rigging or elevation angle, but this mathematically determined prebias is not accurate for all antennas. It is, therefore, desirable to perform near-field measurements on an antenna at the elevation angle at which it is anticipated that the antenna will be utilized, and to be able to map the antenna dish at this angle to determine gravity induced deviations. Adjustments to the panels may then be made to correct for such deviations and the antenna retested at the desired elevation angle to determine if the deviation errors have been reduced within predetermined limits. Several iterations of test and adjustment may be required for high precision antennas, such as those used in radio astronomy, before precision specifications are met. Heretofore, equipment capable of performing near-field measurements on a large antenna at any desired elevation angle has not existed. It has, therefore, not been possible to accurately adjust for gravity-induced antenna deviations in such antenna by performing near-field measurements.
A need, therefore, exist for improved apparatus for performing near-field measurements on antenna and, in particular, for performing such measurements on antenna having large diameter dishes. A need also exist for improved methods of adjusting for gravity-induced deviations in antenna dish panels and, in particular, for such a method which determines deviations from near-field measurements.